The hatch can be opened from the outside, if necessary. The reason for almost closing the hatch is, I believe, to prevent radiative cooling of the cabin. Neither Neil or Buzz remembered any specific reason.

Blackarrow

At some point, when asked this, Neil Armstrong responded along the lines that you shouldn't leave doors or gates (or hatches) open because someone would ask: "Were you born in a field?".

Gonzo

But then again, it's not like anyone ELSE would invite themselves in!

Radiative cooling makes the most sense. Either that or to help keep any kicked up dust out.